The fast-track approval is for people for whom at least two of the most common treatments have failed. These treatments, tyrosine kinase inhibitors or TKIs, include Gleevec (imatinib), Sprycel (dasatinib), and Tasigna (nilotinib).

Although these drugs help most patients, they sometimes fail because of the emergence of drug-resistant CML, or rapidly progressing disease.

"From our perspective, it is another option for patients that might be running out of options," says Hildy Dillon, MPH, senior vice president of patient services for the Leukemia and Lymphoma Society. "I think it will be very meaningful for those who don't do well on the TKIs. With this drug, there is a chance they might respond."

Synribo can have very serious, life-threatening side effects. Some of these side effects, including bone-marrow suppression and bleeding, have been fatal. Other serious side effects include high blood sugar and low blood levels of platelets, red blood cells, and white blood cells.

Synribo is given by under-the-skin injections twice daily for two weeks until white blood cells -- needed to fight infection -- normalize. It's then given for seven consecutive days over a 28-day cycle as long as patients continue to benefit.

Synribo is the semisynthetic version of a Chinese herbal extract derived from an Asian conifer known as the Cowtail Pine or Japanese Plum Yew. This extract, called homoharringtonine or HHT, was considered the most CML-effective treatment, short of bone marrow transplant, back when standard treatments failed for patients in the days before TKIs.